1. Field of the Invention
The present invention relates to a semiconductor laser device. In particular, the present invention relates to a semiconductor laser device in which provision is made for suppressing degradation of a resonator surface caused by optical damage.
2. Description of the Related Art
One of dominant factors which limit reliability of semiconductor laser devices is degradation caused by non-radiative current which is generated in vicinities of resonator surfaces (end facets) when output power is high. Since the number of energy levels produced by surface defects is great at the end facets of the semiconductor crystal layers which realizes resonator surfaces, non-radiative recombination current increases, bandgaps in the vicinities of the end facets are reduced by heat generation which the non-radiative recombination current produces, and absorption regions with regard to laser light are formed. That is, absorption coefficients in the vicinities of the end facets become great, and light absorption and heat generation are enhanced. Thus, the reliability of the semiconductor laser devices is reduced by the heat generation at the resonator surfaces. The conventional techniques proposed for suppressing the non-radiative recombination current (non-radiative current) which causes the above problem can be roughly classified into two techniques.
The first technique is disclosed in the Japanese Patent Publication No. 58(1983)-05320. According to the first technique, the non-radiative current is suppressed by forming a so-called window structure. That is, in order to prevent light absorption in vicinities of end facets and reduce non-radiative current in a semiconductor laser device, a structure being made of semiconductor materials and realizing a bandgap greater than an oscillation wavelength is formed by changing compositions of the semiconductor materials in the vicinities of the resonator surfaces of the semiconductor laser device.
The second technique is disclosed in the U.S. Pat. No. 5,144,634. According to the second technique, a passivation layer made of, for example, silicon is formed after removal of an oxidation layer on a resonator surface of a semiconductor laser device which is produced by cleavage, in order to passivate dangling bonds in compound semiconductors. In addition, a dielectric layer is formed on the passivation layer. Thus, the number of lattice defects generated at compound semiconductor interfaces and non-radiative current can be reduced. Further, the U.S. Pat. No. 5,144,634 also discloses a technique for preventing generation of the above oxidation layer by forming a protection film immediately after cleavage in a vacuum, instead of removing the oxidation layer.
However, in the case where a window structure is formed as disclosed in Japanese Patent Publication No. 58(1983)-05320, the manufacturing process is complex, and correspondingly complex equipment is required. For example, the manufacturing process includes a mask production process for processing only window regions (vicinities of resonator surfaces), an ion injection process for injecting ions, e.g., silicon ions, and an annealing process for realizing thermal diffusion after the ion injection.
On the other hand, in the case where a passivation layer is formed as disclosed in the U.S. Pat. No. 5,144,634, it is necessary to realize at first a state in which no oxidation layer remains on the resonator surface. In order to realize such a state, the oxidation layer on the resonator surface is removed by using a dedicated ion gun. Thereafter, the passivation layer is formed. However, an ultra-high vacuum system is necessary for preventing oxidation of semiconductor surfaces after the removal of the oxidation layer until the formation of the passivation layer, and therefore the vacuum system per se becomes large and expensive.
In the case where the cleavage is performed in a vacuum, the process for removing the oxidation layer can be dispensed with. However, in order to perform cleavage in a vacuum, an apparatus for conveying, cleaving, and coating a wafer in a vacuum is necessary. In addition, in order to adapt such an apparatus for mass production, the construction of the apparatus becomes further complex. Nevertheless, a simple internal structure is required for maintaining an ultra-high vacuum. That is, the complex construction of the apparatus for performing the various processing in a vacuum does not satisfy the requirement for maintaining the ultra-high vacuum.
The object of the present invention is to provide a semiconductor laser device in which non-radiative current generated in a vicinity of a resonator surface is surely reduced, and which is highly reliable and easy to produce.
According to the present invention, there is provided a semiconductor laser device comprising a stack of a plurality of semiconductor layers which include: a substrate; a first cladding layer of a first conductive type, formed above the substrate; an electric-to-optical conversion layer formed above the first cladding layer; and a second cladding layer of a second conductive type, formed above the electric-to-optical conversion layer. In the semiconductor laser device, resonator surfaces are formed at opposite ends of the stack, and an end facet of the electric-to-optical conversion layer at each of at least one of the opposite ends of the stack protrudes outward from the shortest current path between end facets of the first cladding layer and the second cladding layer at the end of the stack through semiconductor layers located between the first cladding layer and the second cladding layer.
In the above construction, carriers flow from the cladding layers into an inside region of the electric-to-optical conversion layer instead of the vicinity of an end facet of the electric-to-optical conversion layer. Therefore, non-radiative current flowing in the vicinity of the end facet can be effectively suppressed, and thus it is possible to prevent degradation of the end facet and achieve high reliability. In addition, the process for forming the resonator surfaces is easier than the processes for producing the conventional semiconductor laser devices in which the aforementioned provision for preventing degradation of end facets is made. Therefore, the manufacturing cost can be reduced by the present invention.
Preferably, the semiconductor laser device according to the present invention may also have one or any possible combination of the following additional features (i) to (xi).
(i) The semiconductor laser device according to the present invention may further comprise at least one protection film with which at least one of the resonator surfaces at the at least one of the opposite ends of the stack is coated. The at least one protection film has a function of controlling the reflectance at the at least one of the resonator surfaces.
(ii) The plurality of semiconductor layers may further include: a first optical waveguide layer formed between the first cladding layer and the electric-to-optical conversion layer; a second optical waveguide layer formed between the second cladding layer and the electric-to-optical conversion layer; and a contact layer formed above the second cladding layer.
(iii) The end of the electric-to-optical conversion layer may protrude outward from the ends of the first optical waveguide layer and the second optical waveguide layer, at the end of the stack.
(iv) In the semiconductor laser device according to the present invention having the additional feature (iii), the end of the electric-to-optical conversion layer and the ends of the first and second optical waveguide layers may protrude outward from the ends of the first cladding layer and the second cladding layer, at the end of the stack.
(v) In the semiconductor laser device according to the present invention having the additional feature (ii), the plurality of semiconductor layers may further include: a first barrier layer formed between the first optical waveguide layer and the electric-to-optical conversion layer; and a second barrier layer formed between the second optical waveguide layer and the electric-to-optical conversion layer.
(vi) In the semiconductor laser device according to the present invention having the additional feature (v), the end of the electric-to-optical conversion layer may protrude outward from the ends of the first and second barrier layers, at the end of the stack.
(vii) In the semiconductor laser device according to the present invention having the additional feature (v), the end of the electric-to-optical conversion layer and the ends of the first and second barrier layers may protrude outward from the ends of the first and second optical waveguide layers, at the end of the stack.
(viii) In the semiconductor laser device according to the present invention having the additional feature (v), the end of the electric-to-optical conversion layer, the ends of the first and second barrier layers, and the ends of the first and second optical waveguide layers may protrude outward from the ends of the first cladding layer and the second cladding layer, at the end of the stack.
(ix) In the semiconductor laser device according to the present invention having the additional feature (ii), the end of the electric-to-optical conversion layer may protrude outward from the end of one of the first cladding layer and the second cladding layer at the end of the stack, and may be recessed from the end of the other of the first cladding layer and the second cladding layer, at the end of the stack.
(x) In the semiconductor laser device according to the present invention having the additional feature (i), at least one first semiconductor layer out of the plurality of semiconductor layers has an end recessed from the end of the electric-to-optical conversion layer at the end of the stack, at least one second semiconductor layer out of the at least one first semiconductor layer is located nearest to the electric-to-optical conversion layer among the at least one first semiconductor layer, and one of the at least one second semiconductor layer has an end recessed from the end of the electric-to-optical conversion layer by the smallest amount of recession among the at least one second semiconductor layer. At this time, the smallest amount of recession may satisfy a condition axe2x89xa6xcex94Dxe2x89xa6xcex4, where a is a lattice constant of the electric-to-optical conversion layer, xcex4 is a thickness of one of the at least one protection film at the end of the stack, and xcex94D is the smallest amount of recession.
In the case where more than one first semiconductor layer out of the plurality of semiconductor layers has an end recessed from the end of the electric-to-optical conversion layer at the end of the stack, and two second semiconductor layers out of the more than one first semiconductor layer are located nearest to the electric-to-optical conversion layer among the more than one first semiconductor layer, and the amounts of recession of the two second semiconductor layers are different, one of the two second semiconductor layers which has an end facet recessed from the end facet of the electric-to-optical conversion layer by the smaller amounts of recession is the one of the at least one second semiconductor layer.
In the case where the amounts of recession of the above two second semiconductor layers are identical, either of the two second semiconductor layers can be the one of the at least one second semiconductor layer.
On the other hand, in the case where only one semiconductor layer out of at least one semiconductor layer which has an end recessed from the end of the electric-to-optical conversion layer at the end of the stack is located nearest to the electric-to-optical conversion layer, the only one semiconductor layer is the one of the at least one second semiconductor layer.
(xi) In the semiconductor laser device according to the present invention having the additional feature (x), when the at least one first semiconductor layer includes more than one semiconductor layer, and two semiconductor layers out of the more than one semiconductor layer are located nearest to the electric-to-optical conversion layer among the more than one semiconductor layer, and one of the two semiconductor layers has an end recessed from the end of the electric-to-optical conversion layer by a larger amount of recession between the two semiconductor layers, the larger amount of recession may satisfy a condition axe2x89xa6xcex94Dxe2x89xa6xcex4/k, where k=xcex94d/xcex94D, and xcex94d is the larger amount of recession.
When the above two semiconductor layers have an end recessed from the end of the electric-to-optical conversion layer by an identical amount of recession, xcex94d=xcex94D and therefore k=1.
When the above condition (x) or both of the conditions (x) and (xi) are satisfied, the non-radiative current flowing through the vicinities of the end facets can be further effectively suppressed or shut off, and a sufficient effect of end facet protection can be obtained. Thus, the reliability of the semiconductor laser device according to the present invention can be further increased.